1. Field of the Invention
The present invention relates in general to digital video cassette recorders, and more particularly to a method of correcting errors in a digital video cassette recorder (VCR) in which the error correcting capability is maximized and probabilities that no error is detected and data decoding fails are minimized.
2. Description of the Prior Art
It is common that an error correcting coder is used in a digital VCR to correct errors occurring in compressing data to record it or decoding the recorded data. In the error correcting coder, there have generally been used block codes or convolutional codes. In particular, the block codes have more widely been used in the digital VCR than the convolutional codes.
In a reed solomon (RS) code as one of the block codes, there are present a part designating input data and a part designating a redundancy. Here, the capability of detecting and correcting the errors is dependent on the redundancy.
In order to maximize the capability of correcting the errors of the data recorded on a tape, it is preferred to use the reed solomon code with a two-dimensional structure as shown in FIG. 1 rather than a one-dimensional structure of a block unit.
Referring to FIG. 2, there is shown a block diagram of a conventional reed solomon coder with the two-dimensional structure. As shown in this drawing, the conventional reed solomon coder comprises a vertical encoder 1 for encoding input data Din in a vertical direction and storing the encoded data in a first memory 2, a horizontal encoder 3 for encoding the data stored in the first memory 2 in a horizontal direction and recording the encoded data on a tape 4, and a horizontal decoder 5 for decoding data played back from the tape 4 in the horizontal direction and storing the decoded data in a second memory 6. The horizontal decoder 5 is adapted to detect errors from the data played back from the tape 4 and correct the detected errors. Also, the horizontal decoder 5 serves to erase all horizontal code sections of the data from which detectable but non-correctable errors are detected.
The conventional reed solomon coder also comprises a second memory 6 for storing output data from the horizontal decoder, and a vertical decoder 7 for decoding the data stored in the second memory 6 in the vertical direction and correcting the data code sections erased in the horizontal direction by the horizontal decoder 5 in the vertical direction.
FIG. 3 is a flowchart illustrating a conventional method of detecting and correcting the errors of the data recorded on the tape in FIG. 2.
The operation of the conventional reed solomon coder with the above-mentioned construction will hereinafter be described with reference to FIG. 3.
In operation, the input data Din is encoded in the vertical direction by the vertical encoder 1 and then stored in the first memory 2. Then, the data stored in the first memory 2 is encoded in the horizontal direction by the horizontal encoder 3 and then recorded on the tape 4.
Here, the errors occurring in the data which is recorded on the tape in the above manner are detected and corrected by the horizontal decoder 5 and the vertical decoder 7 as shown in FIG. 3. First, the horizontal decoder 5 detects the horizontally occurring errors from the data recorded on the tape 4 and corrects the detected errors. Also, the horizontal decoder 5 erases all the horizontal code sections of the data from which the detectable but non-correctable errors are detected. Thereafter, the vertical decoder 7 corrects the data code sections erased in the horizontal direction by the horizontal decoder 5 in the vertical direction.
In the reed solomon code (n1,k1) of the two-dimensional structure, provided that the total length of a code word is n1, a length of a redundancy constituting a portion of the code word is n1-k1, the number of the errors to be corrected is Ec and the number of the errors to be detected is Ed, the error detection and correction can performed within the range of the following equation: EQU Ec+Ed&lt;n-k
Namely, the correction is made possible for the errors of the number smaller than or equal to Ec, but impossible for the errors of the number greater than Ec. The detection is made possible for the errors of the number smaller than or equal to Ed, but impossible for the errors of the number greater than Ec. Also, the horizontal code sections of the data from which the detectable but non-correctable errors are detected are all erased.
In the reed solomon code (n2,k2) of the two-dimensional structure, where n2 is the total length of a code word and n2-k2 is a length of a redundancy constituting a portion of the code word, the data code sections erased in the horizontal direction are corrected in the vertical direction by the vertical decoder 7. Namely, assuming that the number of the vertically existing errors of the data recorded on the tape 4 is t1 and the number of the data sections erased by the horizontal decoder 5 is t2, the error detection and correction can performed within the range of the following equation: EQU t1+2t2&lt;n2-k2
Noticeably, the data sections erased by the horizontal decoder 5 are given the correction capability twice as much as the errors with no position information.
However, the conventional reed solomon coder has a disadvantage in that the horizontal code sections of the data from which the detectable but non-correctable errors are detected are all erased and then corrected in the vertical direction. Namely, when no error is detected from the horizontal code sections of the data, the error correction is made impossible and the non-corrected errors exert a bad influence on a picture quality. Also, because the horizontal code sections of the data from which the detectable but non-correctable errors are detected are all erased, even data sections with no error are erased, resulting in a loss in the information. The information loss means that the errors to be corrected by the vertical decoder is increased in number. Further, the errors which cannot be corrected by the vertical decoder are applied to an error concealment unit, which corrects the surrounding values into estimated values. In this case, an increase in the number of the errors being applied to the error concealment unit results in a degradation in the picture quality.